The Global Positioning System (GPS) includes a constellation of earth orbit satellites transmitting L band signals to GPS receivers using code division multiple access (CDMA) codes. The GPS transmitting satellites use CDMA direct sequence spectrum spreading and the GPS receivers use despreading to acquire encoded data signals transmitted on the two L1 and L2 band signals. These direct sequence CDMA codes are tracked and derive pseudo range estimates used in estimating a navigation solution. Exemplar NRZ formatted quadriphase modulated transmitter and receiver systems are respectively shown in FIGS. 1A and 1B. Spectrum spreading during transmission and spectrum despreading during reception enables reliable reception of the embedded data signals for providing the pseudo range estimates. The exemplar prior art transmitter and receiver systems of FIGS. 1A and 1B use GPS signals having related spectral densities as shown in FIG. 2A, by quadriphase signaling shown in FIG. 2B, using modulators, such as an exemplar L1 QPSK modulator shown in FIG. 2C, all of which use reference designations 10-99.
Referring to FIG. 1A, a GPS transmitter system typically includes an uplink antenna 10 receiving S band signals providing orbital, almanac and ephemeris data to a command and telemetry subsystem 12 which communicates ephemeris data 11a to the baseband processor 13. The ephemeris data at 50 bits per second (bps) 11a is communicated to a baseband processor 13. The clock generator 14 includes an internal reference clock which is used to generate various required clocking signals. The clock generator 14 provides the data clocking signal 11b to the baseband processor 13, and, provides chip rate clock signals 15a and 15b to the baseband processor 13. The clock generator 14 also provides an L1 carrier signal 16a and an L2 carrier signal 16b.
The baseband processor 13 provides a Q phase spread spectrum signal 17a and an I phase spread spectrum signal 17b to quadriphase shift keyed (QPSK) modulators 18 and 20. The Q phase and I phase spread spectrum signals 17a and 17b modulate the Q phase and the I phase of the resulting transmitted quadriphase signal. The spread spectrum signals 17a-b are digital signals having only two levels, for example, plus one and minus one. The spread spectrum signals 17a and 17b modulate the L1 and L2 frequency carrier signals 16a and 16b using modulators 18 and 20 respectively. Both L1 and L2 carrier signals 16a and 16b are modulated in phase quadrature by both of the C/A and P(Y) code spread spectrum signals 17a and 17b, respectively. The L1 carrier signal 16a has a frequency of 1.57542 GHz and the L2 carrier signal has a frequency of 1.2276 GHz. The modulator 18 provides an L1 band signal 25a which is amplified by a high power amplifier 22. The modulator 20 provides an L2 band signal 25b which is amplified by a high power amplifier 24. The Q phase quadrature modulated signal and the I phase modulated signal are modulated onto the respective L1 and L2 carriers 16a and 16b, respectively, in each of the modulators 18 and 20. As shown, both the L1 and L2 band signals 25a-b may include a P(Y) I phase modulated signal and the C/A Q phase modulated signal. Currently, the L1 band signal 25a includes a P(Y) I phase modulated signal and the C/A Q phase modulated signal, and the L2 band signal 25b includes only the P(Y) I phase modulated signal only using a binary phase shift keyed (BPSK) modulator, not shown.
The L1 and L2 band signals 25a-b are then summed together with an L3 band signal 25c using a triplexer 26 connected to a transmitting antenna 27 for transmitting all of the L band downlink signals 25a-b-c. The triplexer 26 receives the L3 band signal 25c from a nuclear detection system 28 using an L3 carrier signal 29 at 1.38105 GHz from the clock generator 14 for generating the L3 band signal also transmitted by the transmitting antenna 27. The L1 and L2 band signals are characterized as NRZ formatted, quadriphase shift keyed signals. The necessary bandwidth is 24 MHz centered around the L1 and L2 carriers 16a-b.
The baseband processor 13 receives clear access (C/A) data and precision (P(Y)) data from the command and telemetry subsystem 12. The C/A data and P(Y) data typically have the same ephemerides data communicated at fifty bps. Respective differing data at differing bit rates could be transmitted as well. The baseband processor 13 also receives necessary chip rate clocking signals 15a and 15b from the clock generator 14 to produce the Q phase and I phase spread spectrum signals 17a and 17b. There is a fixed number of chips per data bit. The baseband processor 13 generates two data modulated direct sequence spreading codes, namely the clear access C/A code and the precision P(Y) code for generating the Q phase and I phase spread spectrum signals 17a and 17b.
The C/A code is generated by a C/A code generator 30 at 1.023 mega chips per second (MCPS) chipping rate. The C/A code is formatted by non-return-to-zero (NRZ) formatter 32. The C/A data 34 is received and NRZ formatted by formatter 36 both clocked using the fifty bps clock signal 11b. This NRZ formatting is commonly referred to as NRZ formatting. A C/A code spreader 38, which functions as a multiplication mixer, spreads the NRZ formatted C/A data from formatter 36 with the NRZ formatted C/A code from formatter 32, to provide the Q phase spread spectrum signal 17a. The C/A code generator 30 is clocked by the 1.023 MCPS chip rate clock signal 15a and provides the C/A spread spectrum code that spreads the spectrum of the fifty bps NRZ formatted data 36 with the NRZ formatted C/A code. The C/A code is a well known Gold code.
Likewise, the P(Y) code is generated by a P(Y) code generator 40 but at 10.23 MCPS of the P(Y) chip rate clock signal 15b, which is ten times the chipping rate of the C/A code chip rate clock signal 15a. The P(Y) code is also formatted by a non-return-to-zero (NRZ) formatter 42. The P(Y) data 44 is received and NRZ formatted by formatter 36 using the fifty bps clock signal 11b. A P(Y) spreader 48, which functions as a mixer, spreads the NRZ formatted P(Y) data from formatter 46 with the NRZ formatted P(Y) code from formatter 42, to provide the I phase spread spectrum signal 17b. The P(Y) code generator 40 provides the P(Y) spread spectrum code that spreads the spectrum at a chip rate of 10.23 MCPS. The P(Y) code is an encrypted code for military use.
Referring to FIGS. 1A and 1B, and more particularly to FIG. 1B, a GPS receiver system typically includes a receiving antenna 51 to receive the L band signals 25a-b. The GPS receiver system receives the L band signals 25a-b from the GPS satellite transmitter system of FIG. 1A, but is shown to be configured to despread only one of the L1 or L2 band signals 25a or 25b for clarity and convenience. The L band signals 25a-b are received by the antenna 51 and then amplified by a low noise amplifier 52 providing a received L band signal 53. The L band signal 53 is despread by a C/A code despreader 54 and a P(Y) code despreader 56 using the formatted C/A code and P(Y) code, respectively. The C/A code is generated by a C/A code generator 58 connected to an NRZ formatter 60 using the 1.023 MCPS chip rate clocking signal 61. The C/A NRZ formatter 60 provides a formatted C/A code signal communicated to the C/A code despreader 54. The P(Y) code is generated by a P(Y) code generator 62 connected using a 10.23 MCPS P(Y) chip rate clocking signal also communicated to an NRZ formatter 64 providing a formatted P(Y) code signal communicated to the P(Y) despreader 56. The despreaders 54 and 56 provide despread C/A signal 55 and a despread P(Y) signal 57, respectively, to a code tracker 66 and carrier tracker 65. The code tracker 66 performs conventional code tracking using the L band signal 53 to synchronize on the received code and provides the 1.023 MCPS C/A and 10.23 MCPS P(Y) chip rate clock signals 61 and 63 for despreading the C/A and P(Y) code signals on the L band signal 53.
The carrier tracker 65 uses the despread signals 55 and 57 to demodulate the despread signals 55 and 57 on the L1 or L2 carrier signals. The carrier tracker 65 includes conventional tracking loops 70 providing orthogonal carrier signals 71a and 71b and communicated to a C/A demodulator 72 and a P(Y) demodulator 74 respectively providing C/A and P(Y) demodulated signals 73 and 75. The carrier tracker 65 enables demodulation by providing orthogonal carrier signals 71a and 71b that are in phase with the same respective carrier of the despread signals 55 and 57. The carrier signals 16a-b of the transmitter system are the same frequency as the carrier signal of the received L band signal 53 of the receiver system. The function of the tracking loops 70 is to demodulate the received carrier from the despread signals 55 and 57. The tracking loops 70 may be conventional Costas or squaring loops that maintain frequency and phase coherence of the orthogonal carrier signals 71a and 71b for demodulation of the L1 and L2 carrier signals 16a and 16b from the despread signal 55 and 57, respectively.
The C/A demodulated signal 73 is communicated to a C/A data detector 76 and a C/A bit synchronizer 77 and is used for detecting the C/A data 78. The C/A bit synchronizer 77 determines bit boundaries of the C/A demodulated signal 73. The C/A detector 76 is used to detect data bits within NRZ formatted C/A data. The C/A bit synchronizer 77 generates a fifty bps signal clock for the detector 76 for providing the C/A data 78 as a bit stream. Likewise, The P(Y) demodulated signal 75 is communicated to P(Y) data detector 79 and P(Y) bit synchronizer 80 for detecting the P(Y) data 82. The bit synchronizer 80 determines bit boundaries of the P(Y) demodulated signal 75 and generates a fifty bps clock signal for the P(Y) detector 79. The P(Y) detector 79 is used to detect data bits within NRZ formatted P(Y) data bits. The fifty bps signal 67 clocks the P(Y) detector 79 for providing P(Y) data 82 as a bit stream.
Referring to FIGS. 1A-B and 2A-C, and more particularly FIGS. 2A-C, the GPS transmitter and receiver systems are characterized as NRZ formatted quadriphase modulated systems. The spectral density 90 of the transmitted C/A code signal and the spectral density 92 of the transmitted P(Y) code signal are shown at baseband. Only the center spectral lobes are shown for clarity in FIG. 2A, but it is well understood that the spectral densities 90 and 92 have many smaller sidelobes extending well beyond the depicted frequency range from the center carrier frequency. The C/A and P(Y) code signals are on the carrier frequencies, L1 or L2 , so that both can be demodulated by a carrier tracker 65 using despread signals 55 and 57. Currently only the P(Y) signal is modulated on the L2 carrier. The GPS system is based on quadriphase signaling having Q and I orthogonal signaling vectors 96a-d generated by the QPSK modulators 18 and 20.
The C/A data and C/A spreading code are modulated on Q phase vectors 96b-d and the P(Y) data and P(Y) spreading code are modulated on the I phase vectors 96a-c. The L1 QPSK modulator injects a ninety degree phase shift by phase shifter 97 to the unmodulated carrier signal 16a whereas the L2 modulator only uses the I channel. The modulated signals from mixers 98a and 98b are summed by an adder 99 to provide the L1 band quadriphase modulated signal. The orthogonal despread modulated signals 55 and 57 are respectively demodulated by demodulator 72 and 74 using orthogonal carrier signals 71a and 71b.
The C/A code spectral density 90 and the P(Y) code spectral density 92 of a transmitted L band signal have maxima at the carrier frequency. Hence, the C/A and P(Y) code signals have overlapping spectra. The overlapping spectra disadvantageously produces cross coupling between the C/A and P(Y) signals degrading system performance. A single source of interference at the carrier frequency disadvantageously interferes with both the C/A and P(Y) signals deteriorating the reception of either code signal. Hence, the spectral maximum of the signals at the center carrier frequency disadvantageously renders both code signals susceptible to interference operating at the carrier frequency. Spectrum isolation between the C/A and P(Y) code signals is desirable, but operation at the common carrier frequency disadvantageously limits spectral isolation when only NRZ formatting is used.
Any change in the GPS transmission method should be backward compatible with existing formats, modulation and carrier frequencies. One possibility is to change the format and modulation of the C/A code or P(Y) code, however, backward compatibility requires the use of the same format and modulation for the C/A and P(Y) signals. Backward compatibility necessarily suggests using existing NRZ formatters and modulators. Backward compatibility therefore makes it necessary to use the current spectrum allocation more efficiently through some type of frequency reuse or spectrum sharing to satisfy user needs.
Users of GPS have found it desirable to add a third NEW code to be included in the GPS L band allocations. The NEW code should be spectrally isolated from the C/A code signal spectrum. However, the addition of a NEW code in the L band allocations would also render it susceptible to the same interfering signals that would interfere with the C/A and P(Y) signals. The addition of the NEW code should not affect the C/A code and P(Y) code signals retaining backward compatibility.
A number of spectrum sharing techniques, such as, time division multiple access, and, frequency division multiple access, may be considered as a direct approach to achieve improved spectral separation of the C/A code with the NEW code so that only one signal is effectively interfered with by a single interfering signal. Providing the C/A code on the L2 carrier will help satisfy C/A user needs for a second frequency to not only improve the accuracy achievable, but also would standardize the overall satellite navigation architecture by having the C/A and P(Y) code signals carried by both L1 and L2 carriers. With both C/A and P(Y) signals modulating one or both L band carriers, a single interfering signal on each carrier could render both C/A and P(Y) signals unusable. Hence, it would appear desirable to create a NEW carrier for the NEW code signal having sufficient spectral separation and isolation from both the C/A and P(Y) signals. However, the use of yet another carrier would disadvantageously complicate GPS especially when no other bands are available. These and other disadvantages are solved or reduced using the present invention.